By virtue of the structural differences, there are basically two different types of cells that can be considered, namely prokaryotic and eukaryotic.The pro -karyon (or the cell without the karyon or nucleus in Greek) are the bacteria and the cyanobacteria (according to the five kingdom classification by Whittaker(1969) .On the contrary, the eukaryotes demonstrated many developed characteristics than the prokaryotes.
BASIC CHARACTERS OF PROKARYOTES
- Simple organization, smaller size (0.1– 0.25 mm),found in land ,air and water
- A true nucleus with nuclear membrane absent, and is called nucleoid, nucleolus and centrioles absent,no meiotic reduction
- All well defines cytoplasmic organelles like mitochoindria, Golgi bodies, lysosomes,
- Ribosomes remain scattered in the cytoplasm,but DNA is without histones and is a double stranded DNA
- Ribosomes are 50S and 30 S. Jointly 70S
- Cell wall composed of carbohydrate, muramic acid and amino acid, may extend into cytoplasm as mesosome with respiratory enzymes.
- Absence of cytoplasmic movement, locomotion is by flagella that is not multistranded and made of flagellin
- Absence of splicing mechanism as post transcriptional modification, nickase enzyme is absent
- High genetic flexibility and biochemical adaptability.
- Prokaryotes are usually spherical, or comma shaped, or rod or spiral in shape
Besides, the prokaryotes (Bacteria ) are very important for the maintenance of the ecosystem, they not only act as saprophytes but also help in digestion as well as clean oil spills at sea.
Throughout the seventeenth century, different scientists have tried to make their own observations on the structure , function, and the origin of cells. Workers like Nehemiah Grew (1672), to Turpin (1826) were of opinion that all living things are made of cells and they are organized into tissues, with each cell having a sufficiency in its own structure and function. Besides, the discovery of the nucleolus by Wagner ((1832), protoplasm by Purkinjee (1839) provided a new insight to the cell as described by Robert Hooke
However , all of the different facets of the cells origin, structure and function gradually came to be amalgamated into the cell theory as proposed by Mathias J Schleiden (1838) and Theodore Schwann (1839) and it eventually brought to light the cell theory in 1839.
The basic points of cell theory are as follows
- All living organisms are made of cell or cells
- Each cell has its own independent functional capacity
- Each cell originates from a preexisting cell
- Each cell is a carrier of the genetic material
- Cells are the structural and functional unit of a living system
- The properties of an organism is dependent on the properties if the cells that make it
However , the concept of presence of origin of cells from preexisting cells was strongly advocated by Virchow in 1855 Continue reading
To start with, the cell as we obtain it from the cell theory , is perhaps the most important structure that allows our primary inter-phase with the intricacies of the biological world, the complexities in form and function, but the mere reduction in size ,yet forming the building block for a human with trillions of cells.
So how do you define a cell?
In a nutshell it can be defined as the unit of the living system or as an unit that makes up the living body. But if one is a little careful to provide a holistic or complete definition ,then he or she might want to say that it is
protoplasmic mass bounded by a plasma membrane with cytoplasmic organelles and nucleus with DNA (except mature Red blood cells ), that can perform all physiological activities of survival and reproduce(except nerve cells).